Undertones when playing minor 3rds

CorOS Version: 1.4.1

In many of the clean amp models, a very low frequency can be heard when playing an harmonic interval (minor 3rd). For me it was most noticeable when playing the G on the 12th fret on the G-string and the E on the 14th fret of the D-string at the same time. I found it out playing around with the D-Cell models, but then also heard the same phenomenon in other amp models. To eliminate other factors, I checked the grid with only the amp model without any effects or cab simulations, but the low undertones were still there. This effect does not appear to happen on the Neural Captures of amps.

I was wondering if you guys maybe experienced a similar issue.

Hi @TCvanHoek and welcome to the community! That is an odd issue and not one I have heard about since the arrival of the QC. Have you tried another guitar? Almost sounds like you have an odd EQ added or something similar? Worst case, email support@neuraldsp.com and they will likely have you send your logs, possibly backup and complete a factory reset and or analyze your current setup. Either way, unfortunately I have no ideas and haven’t heard any odd undertones since owning the QC for almost 2 yrs now.

Wondering if you are hitting a resonant frequency in your monitor or even something vibrating in the room? Does this happen in your headphones? Different monitor, different room/space?


Thank you for your quick replies. After your comments, I tried it with a different guitar and even with a sine wave as well, it is still there. Also on headphones.
My guess is that it is some kind of aliasing issue in the amp model. If you are interested, I made a soundclip of the phenomenon:

Gibson ES-335 → D-Cell Hisbert Ch2
Sine Wave → D-Cell Hisbert Ch2

I will also share it with support@neuraldsp.com

Thanks for sharing with us/support. Unfortunately, I am unable to open/play your sound-clip.

Maybe it works this way:

That worked! Thanks and that is weird. I tried to duplicate the undertones using that specific amp model and wasn’t successful. Are you using a specific cab or IR? It will be interesting to see what support says or how they go about diagnosing the issue.

Hmm, I used a splitter in my preset to run 2 cab blocks, when I heard this issue I bypassed everything so that only the amp model was active. The splitter/mixer was still there, so maybe that caused the issue?

That is entirely possible. When I tried to duplicate the bug, I used that specific amp (D-Cell Hisbert Ch2) and my goto favorite IR but nothing else on the signal chain. Have you tried without the splitter and just using one cab? Could it be a phase issue with the two specific cabs?

I will add, 1st time I have used that amp in a while and after messing with it for a while, I really like the ch2 model!

Yeah! It is really cool, from clean to crazy drive sounds!

The thing is, I bypassed the cab blocks, so I don’t think it is related to the IRs I was using. I will try replicating the issue with a blank preset when I am home. Then we might be able to pinpoint where the problem comes from


With a grid with only the amp model, it is still there! So it is not caused by the cab blocks, IRs, the splitter or any kind of effect. It is the most noticeable when the gain is turned down.

Is someone else also hearing the same phenomenon on their QC?

Not sure if you posted that you had tested with another guitar yet. If not, try a different guitar. The fact that you mention that the undertones are most noticeable when the guitar is turned down is suspiciously reminiscent of a problem some users report when they are using a pitch shifting block. They can actually hear the note coming directly off their guitar as well as the pitch shifted one from their QC. Could this be the issue?

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Thanks for chipping in! I’ve tried it with several guitars and even a sine wave to rule that out.

Have you listened to the soundclip from the dropbox link?

I have listened to it although I have not tried to replicate your results. At this point if it were me I would probably back up to the cloud and force another firmware upgrade (after swapping out all cables and guitar in my signal chain). If that didn’t work, it is probably time to contact Neural support as @MP_Mod recommended. If it is your particular device or configuration, Neural can fix/replace it, and if not, you will have provided the valuable contribution of bringing a significant bug to their attention.

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I agree 100%. I can’t replicate it but support may have you backup and factory reset as a worst case scenario and or development may become aware of a bug and resolve the issue. It will be interesting to hear the outcome.

I’ve shared the details with NeuralDSP support and I will keep you up-to-date :slight_smile:

Hey, I listened to your sound files and I replicated the exact setup we’re talking about here: D-Cell Hisbert (one of my favorite amp models in the QC!) Ch2, picking the same notes.

I hear what you’re talking about, but I don’t think it’s a problem. Try playing a major third and notice how that interval changes. For reference, I pulled up a handful of Neural plug-ins on my Mac and tried the same thing. The low interval thing is there on all of them.

I pulled up a quick search for undertones and found this Wikipedia article on the undertone series. There are probably more articles that can explain this concept in a less complicated way. If I find something, I’ll post it here.


Maybe this could point you in a different direction than something being wrong with your QC.

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I browsed the Wikipedia article, but they were not very specific about what caused the undertones other than mentioning “dividing a monochord string”. You reported seeing the same results in Neural plugins. I wonder if other companies’ modelers or plugins are also creating these undertones. If so, I find it notable that in all my time on device forums I have never seen anyone else bring this up. Really interesting find by the OP.

The OP’s findings reminded me of some of the material on waves from an engineering physics class I took years ago; that’s why I thought to look it up. I didn’t have time to read the whole article this morning, though. Maybe I can find more on this subject.

EDIT: I’ve got other non-Neural guitar plug-ins too, let me try that out.

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